It is well known that gas turbine engines include a rotor assembly which is rotatable relative to stationary engine structures, including a rotor mounting structure. In the rotor assembly, there are a number of rotatable components, for example; a central shaft, shaft cones, compressor blades and disks, turbine buckets and wheels, and dynamic air seals. These components are each reacted upon by axial pressure forces which may be static or dynamic. The result of a vector sum of these axial pressure forces is a net axial thrust which may by exerted in either of two opposing directions. This net axial thrust places axial loads on the stationary mounting structure. Typically, a rotor thrust bearing is arranged to absorb this load without interfering with the free rotation of the rotor assembly. Commonly this bearing takes the form of a ball bearing, roller bearing or tapered bearing encased within a thrust bearing housing.
During operation of the engine, axial load exerted on the rotor thrust bearing varies as the pressures on the various rotatable components change. Excessive net axial thrust over a prolonged period is a significant cause of wear which can lead to failure of the rotor thrust bearing. As bypass ratios within the turbine increase, axial loads get larger whilst bearing size is reduced. Thus, it becomes problematic to design a bearing of a size which can be accommodated in the turbine and has sufficient capacity to cope with these axial loads. To address this problem, it is known to incorporate into the turbine a thrust balancing mechanism which limits the amount of net axial force imposed on the rotor thrust bearing.
Known thrust balancing mechanisms adopt a range of design principles. For example; increasing internal seal diameters, using higher internal pressures, and adding “spring pack” bearings. Each of these principles is effective in addressing the problem but at a trade-off to other turbine design parameters. Increasing seal diameters increases air system losses and adds weight; using higher internal pressures increases air system losses and “spring pack” bearings add cost and weight.
Known arrangements operate to provide a fixed load to counter net axial thrust, however, since the net axial thrust varies during a flight cycle, these fixed loads can result in unwelcome net loads at times during the engine cycle.